For example, as a rotational-to-linear motion conversion mechanism, one described in Patent Document 1 is known.
This conversion mechanism includes a pipe-shaped ring shaft, a sun shaft disposed inside the ring shaft to extend coaxially with the ring shaft, and a plurality of planetary shafts disposed between the sun shaft and the ring shaft to extend in the same direction as do the sun shaft and the ring shaft. A female thread is formed on the inner circumference of the ring shaft and a male thread is formed on the outer circumference of the sun shaft and on the outer circumference of the planetary shaft. The male thread of the planetary shaft is mated with the male thread of the sun shaft and the female thread of the ring shaft.
In the conversion mechanism configured as above, a rotational motion of the ring shaft will cause the planetary shaft to make a planetary motion, i.e., to rotate about its own axis while rotating about the axis of the sun shaft. The planetary motion of the planetary shaft in turn produces an axial linear motion of the sun shaft. The aforementioned conversion mechanism thus converts the rotational motion of the ring shaft to the linear motion of the sun shaft.    Patent Document 1: WO2004/094870 A1
In a process of fabricating the rotational-to-linear motion conversion mechanism, the inventor found that the planetary shaft is tilted between the ring shaft and the sun shaft from the parallel position (i.e., a position in which the axis of the planetary shaft is parallel to the axis of the sun shaft). The planetary shaft is possibly tilted in this manner for the following reason.
In the conversion mechanism, the number of threads of each component is set to a different value. Accordingly, engagements between the female thread of the ring shaft, the male thread of the sun shaft, and the male thread of each planetary shaft would produce backlash therebetween. The magnitude of backlash may vary depending on the setting of each number of threads. For this reason, in the process of fabricating the conversion mechanism, force acting radially upon the planetary shaft would cause the planetary shaft to displace and tilt with respect to the sun shaft by the amount of the backlash. Under this condition, each component of the conversion mechanism is fabricated.
In the conversion mechanism having the planetary shaft tilted with respect to its parallel position, the threads of each component are mated unevenly with each other, thereby accelerating local wearing of the threads with loss of service life. This also causes an increase in friction between the components, resulting in a decrease in the efficiency of conversion from rotational to linear motion.